libafs: Abstract the Linux lru cache interface

libafs: Abstract the Linux lru cache interface

Define static functions afs_lru_cache_init, afs_lru_cache_add and
afs_lru_cache_finalize to handle interfacing with Linux's lru
facilities.

This change's primary purpose is to isolate the preprocessor
conditionals associated with the details of the system lru interfaces to
just these functions and to simplify the areas that utilize lru caching
by removing the preprocessor conditionals.

As Linux's lru facilities change, additional conditional code will be
needed.

Reviewed-on: https://gerrit.openafs.org/14167
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit dca95bcb7efdff38564dcff3e8f4189735f13b3a)

Change-Id: I863bbc9bb578716c42fdf34672ec8ad85f05ea31
Reviewed-on: https://gerrit.openafs.org/14209
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>